The electrolytic production of chlorine and caustic by the electrolysis of brine has been well known for many years. Historically, diaphragm cells using a hydraulically-permeable asbestos diaphragm, vacuum-deposited onto foraminous steel cathodes, have been widely commercialized. Such diaphragm cells, employing permeable diaphragms, produce NaCl-containing NaOH catholytes because NaCl passes through the diaphragm from the anolyte to the catholyte. Such NaCl-containing caustic generally requires a de-salting process to obtain a low-salt caustic for industrial purposes.
In recent years, the chlor-alkali industry has focused much of its attention on developing membrane cells to produce low-salt or salt-free caustic in order to improve quality and avoid the costly desalting processes. Membranes have been developed for that purpose which are substantially hydraulically-impermeable, but which will permit hydrated Na.sup.+ ions to be transported from the anolyte portions, while substantially preventing transport of Cl ions. Such cells are operated by flowing a brine solution into the anolyte portion and by providing salt-free water to the catholyte portion to serve as the caustic medium. The anodic reactions and cathodic reactions are the same regardless of whether a membrane cell or a diaphragm cell is employed.
Since the disclosure of fluorocarbon polymers containing sulfonic acid functional groups (U.S. Pat. No. 3,282,875), a great deal of progress has been made in adapting the polymers for use as membrane materials in electrochemical cells. The bulk of this work has dealt with the production of chlorine and caustic soda. The membrane materials are also applicable to caustic potash and as separators for a variety of other membrane separator applications. In chlor-alkali applications, the membrane materials can be used in cells which produce hydrogen as a co-product of the cathodic reaction producing caustic soda or they may be used in chlor-alkali cells which contain depolarized cathodes (U.S. Pat. No. 4,035,254 and U.S. Pat. No. 4,035,255); cells where caustic soda is produced without the attendent production of hydrogen.
In general, the polymers have found most widespread use in the above applications when the functional group is on a fluorocarbon chain which is pendant to the main polymer backbone. Fluorocarbon sulfonic acid polymers and carboxylic acid polymers have been disclosed which have the functional group attached directly to the backbone, but these polymers have found scant utility (U.S. Pat. No. 3,041,317 and Brit. Pat. No. 1,497,748). The polymer materials, whether fluorocarbon sulfonic acids or carboxylic acids, have in general been made by copolymerizing monomers such as tetrafluoroethylene or chlorotrifluoroethylene with fluorocarbon vinyl ethers which contain an acid or an acid precursor functional group (U.S. Pat. No. 3,282,875 and Brit. Pat. No. 1,518,387).
The function of a membrane in a chlor-alkali cell is to separate the product(s) produced at the cathode, from the products and salt in the anolyte compartment. The membrane must effect this separation at low electrical resistance to be economical. Three factors prevail in measuring the efficiency at which a given membrane performs: (1) current efficiency is a measure of how well the membrane prevents hydroxide ion formed in the catholyte chamber from migrating to the anolyte chamber; (2) caustic concentration in the catholyte chamber is important because water is evaporated from the cell product; and (3) cell voltage, which reflects the electrical resistance of the membrane among other things, determines the power requirements of operating the cell.
A relationship between water absorption of the polymer and usefulness of the polymer as a membrane has long been recognized (W. G. F. Grot, et al, Perfluorinated Ion Exchange Membranes, 141st National Meeting The Electrochemical Society, Houston, Tex., May, 1972). Grot disclosed that the capacity of the polymer to absorb water is a function of the equivalent weight, the history of pretreatment of the polymer and the electrolytic environment of the polymer. The equivalent weight is the weight of polymer which will neutralize one equivalent of base. A standard method of measuring water absorption for meaningful comparisons is given in Grot's paper (above). The method consists of boiling the polymer for 30 minutes in water with the polymer being in the sulfonic acid form. The water absorbed by the polymer under these conditions is called the "Standard Water Absorption". The sulfonic acid membranes reported on in Grot's paper are polymers disclosed in U.S. Pat. No. 3,282,875.
Since the original disclosure of the sulfonic acid fluoropolymers, and particularly in recent years, functional groups other than sulfonic acid have received considerable attention. In fact, it has been stated that, because of excessive hydration, sulfonic acid groups cannot be used for making caustic above about 18% directly in the cell (M. Seko, "Commercial Operation of the Ion Exchange Membrane Chlor-Alkali Process", The American Chemical Society Meeting, April, 1976, New York, N.Y.). While high caustic strength is desirable, as a general rule, membranes capable of high current efficiency at high caustic concentration have higher electrical resistance. Thus, even though one carboxylic acid membrane is capable of producing in excess of 30% caustic at greater than 90% current efficiency, actual operation at 21.6% caustic was found to be more economical because of excessive electrical resistance at the higher caustic strengths. (M. Seko, "The Asahi Chemical Membrane Chlor-Alkali Process", The Chlorine Institute, Inc., 20th Chlorine Plant Managers Seminar, New Orleans, Feb., 1977). In addition to the many efforts to achieve higher caustic strength using carboxylic acid membranes, there has been a great deal of effort to achieve the same results with sulfonamides (U.S. Pat. No. 3,784,399). Again, higher caustic strength is achieved only at the expense of greater power consumption which is caused by higher electrical resistance of the membrane. In addition, membranes capable of higher caustic strengths are more adversely effected by impurities which enter the cell with the incoming salt feed. Thus, useful operating life of these membranes is generally less than sulfonic acid membranes.
One way of correlating functional groups to performance is to measure water of hydration per functional group in the polymer. Carboxylic acid polymers (U.S. Pat. No. 4,065,366) and sulfonamide polymers hydrate less than sulfonic acid polymers, (C. J. Hora, et al., Nafion.RTM. Membranes Structured for High Efficiency Chlor-Alkali Cells, 152nd National Meeting The Electrochemical Society, Atlanta, GA., October, 1977) where the polymer structures are comparable. Changes in functional group concentration in a given polymer structure results in changes in the hydration water per functional group. Thus Hora disclosed that a 1500 eq. wt. sulfonic acid polymer of given structure has less water of hydration per functional group and operates at higher current efficiency than an 1100 eq. wt. polymer of the same general structure. In turn, the electrical resistance of the 1500 eq. wt. material is higher than the 1100 eq. wt. material because of fewer sites to transport ions and thus to conduct current. Sulfonic acids membranes which are useful in chlor-alkali cells are taught to have eq. wts. in the range of 1100 to 1500. In practice, eq. wts. of 1500 and 1600 are considered best for preventing migration of hydroxide ions from the catholyte to the anolyte without unreasonable cell voltage penalties (C. J. Hora, et al, Nafion.RTM. Membranes Structured for High Efficiency Chlor-Alkali Cells, 152nd National Meeting The Electrochemical Society, Atlanta, GA., October, 1977). Extremely thin films of these materials are required to meet the voltage requirements.
Data for water absorption of sulfonic acid polymers and sulfonamide polymers has been published by Hora and Maloney in the above publication. In this paper, the polymer structures are the same except for the substitution of sulfonamide groups for sulfonic acid groups. The data shows that, for given eq. wts., the sulfonamides absorb only 35-60% as much water as do sulfonic acids. A particular case shown is a comparison of 1200 eq. wt. membranes. There, the sulfonic acid membrane absorbs about 20 moles of water per equivalent of sulfonic acid, while the sulfonamide, from methylamine, absorbs 12.3 moles of water per equivalent of sulfonamide and the sulfonamide, from ethylenediamine, absorbs only 8.1 moles of water per equivalent of sulfonamide. From another paper (H. Ukihashi, Ion Exchange Membrane For Chlor-Alkali Process, Abstract No. 247, American Chemical Society Meeting, Philadelphia, April, 1977) it can be calculated that a carboxylic acid membrane having an eq. wt. of 833 absorbs 8.3 moles of water per equivalent of carboxylic acid and that another having an equivalent weight of 667 absorbs 9.2 moles of water per equivalent of carboxylic acid.
In addition to the work described above where means for increasing caustic concentration in operating cells by using membranes having functional groups that hydrate less than sulfonic acids are used, methods for operating the cells, themselves, that lead to increased caustic concentration have been reported. Thus, series catholyte flow (U.S. Pat. No. 1,284,618) and series catholyte and anolyte flow (U.S. Pat. No. 4,197,179) can lead to increased caustic strength without sacrificing either current efficiency, cell voltage or membrane life. In addition, there are numerous applications where high strength caustic is not needed. In the case of caustic produced by diaphragm cells, evaporation is necessary to remove the salt in the caustic. This is not necessary with caustic produced in membrane cells. The only need for evaporation then becomes a matter of the few applications requiring high strength product and cases where the product is to be shipped long distances. Evaporation to high strength, such as 50%, reduces the volume to be shipped and, depending on the distance of shipment, can be more economical. Thus, it can be seen that in many applications and when series flow methods of cell operation are used, sulfonic acid membranes of the prior art and certainly improved sulfonic acid membranes are of great value.
U.S. Pat. No. 4,025,405 show electrolytic cells having a stable, hydrated, selectively permeable, electrically conductive membrane. The membrane is a film of fluorinated copolymer having pendant sulfonic acid groups containing recurring structural units of the formula: ##STR1## in which R' is F or perfluoroalkyl of 1 to 10 carbon atoms; Y is F or CF.sub.3 ; m is 1, 2, or 3; n is 0 or 1; X is F, Cl, H, CF.sub.3 ; X' and X are CF.sub.3 --(CF.sub.2).sub.z wherein Z is 0-5; the units of formula (1) being present in an amount of from 3-20 mole percent.
In addition to development of sulfonic acid fluoropolymers for use as membranes in electrolytic cells, the acid form of the polymers have received extensive interest as solid superacids. In general these materials have been used as strong acid catalyst for organic reactions. It has been reported that the polymers of the prior art are useful for alkylation of aromatics with olefins, alkyl halides, alcohols, esters, and the like as well as esterification, ketal (acetal) formation, Diels-alder reactions, pinacolpinacolone rearrangement and hydration of alkyner (G. A. Olah, New Synthetic Reagents and Reactions, Aldrichimica Acta, Vol. 12, No. 3, 1979).